Upgrading of wood by hydrothermolysis is known in the art. In a hydrothermolysis, wood is treated with saturated steam at a temperature between 130-220° C., wherein a hemicellulose and lignin become reactive. In a subsequent step the wood is cooled and cured by drying, wherein the reactive hemicellulose and lignin form cross-links. The final product is wood which has acquired a greater durability and fungal resistance than the untreated wood. Since all that is required for the reaction is water in the form of steam, hydrothermolysis is particularly advantageous compared to preserving methods in which the wood is upgraded with impregnating agents usually having an environmental impact.
Drawbacks of known hydrothermolysis methods are that the methods are particularly time-consuming. The wood is treated in multiple individual steps with interim cooling and heating of the wood. Time is lost here and the wood is placed under great internal stresses as a result of contraction and expansion, which result in splitting and deformation. This results in high costs due to longer production times on the one hand and to a decrease in the economic value of the wood on the other.
More rapid methods of thermal preservation are known wherein dried wood is heated to very high temperatures above 220 degrees Celsius. The drawback of these rapid methods is, however, that the mechanical strength of the product decreases greatly when compared to the starting products. Rapid thermal treatment methods with less high temperatures result in products with a reduced durability when compared to the products at higher temperatures. The known, relatively rapid methods also produce a high percentage of products of low-grade quality due to splitting and deformation, leading to a relatively high loss of material and a high amount of wasted wood